Container leak testing

ABSTRACT

Apparatus for leak testing a blow molded container comprises an electrically conductive probe for extending into the interior of the container through an opening and electrically conductive earth plates for surrounding at least a major portion of the container. In order to test the container, the probe is introduced into the container by an actuator, the earth plates are placed in close proximity with the container walls by means of actuators and a high voltage is applied to the probe by way of a connecting lead. A current sensor is responsive to the occurrence of a spark between the probe and the earth plates thereby indicating the presence of a hole or weakness in the intervening container wall. Such apparatus can be implemented at relatively low cost and is highly reliable in practice, as well as requiring only a short testing cycle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to leak testing of containers, and isparticularly, but not exclusively, concerned with the leak testing ofblow molded containers.

2. Description of the Related Art

It is an inherent feature of the blow molding process used for producingplastics bottles and the like that small holes are sometimes formed inthe container walls due to the occurrence of flaws in the moldingmedium. It is therefore essential, in applications in which thecontainer is required to be fluid tight, that each container issubjected to leak testing prior to use. Conventionally such leak testingis performed on a conveyor line by initially pressurising each containerwith air and subsequently detecting by means of a pressure transducerany decay of the pressure within the container which might indicate theoccurrence of a leak. While this leak method operates satisfactorily inpractice, it requires use of a pressure transducer which is both costlyand delicate. Also, since the method requires monitoring of the pressurewithin the container over a period of a few seconds, the period of eachtesting cycle cannot be decreased below a certain value.

SUMMARY OF THE INVENTION

It is an object of the invention to provide generally improved apparatusand methods for leak testing such containers.

According to the present invention there is provided apparatus for leaktesting a container comprising an electrically conductive probe forcommunicating with the interior of the container by way of an opening ofthe container, electrically conductive shield means for locationexternally of the container, electrical connection means for applicationof a high voltage between the probe and the shield means, and sensormeans for detecting current flowing between the probe and the shieldmeans indicating the presence of a hole or weakness in the interveningcontainer wall, and characterised in that the electrical connectionmeans is arranged to apply the high voltage to the probe, and thatmoving means is provided for effecting relative displacement of thecontainer and the shield means so as to surround at least a majorportion of the container with the shield means.

Such apparatus can be implemented at relatively low cost and is highlyreliable in practice. It is capable of detecting not only very smallholes in the container wall, but also weaknesses in the container wallprovided that a sufficiently high voltage is used. Since the occurrenceof current flow in the form of a spark is almost instantaneous in theevent of a hole or weakness being detected, the apparatus may beincorporated in a conveyor line and mechanised in such a manner thateach testing cycle takes place in a relatively short period of time. Thespark corresponds to a breakdown of insulation between the twoconductors such as to cause a current discharge between the conductorsunder the applied electric field.

The probe advantageously has a form which ensures that parts of theprobe are located in close proximity to the inside wall of the containerat a plurality of locations during testing. For example the probe mayinclude an inflatable electrically conductive balloon, or a plurality ofelectrically conductive strands capable of spreading out under theinfluence of a static charge, or even a plurality of mechanicallyactuable electrically conductive vanes.

The shield means may comprise electrically conductive plates orelectrically conductive foam pads or electrically conductive brushes forconforming to the external profile of the container. In a furtheralternative the shield means may comprise a plurality of parts which areelectrically insulated from one another and connected to earth (ground)by respective discriminating means so as to enable the approximatelocation of the hole or weakness to be detected by the sensor means.

The moving means preferably comprises a plurality of mechanicalactuators for moving associated parts of the shield means into closeproximity with the outside wall of the container. Each actuator may be apneumatic or hydraulic piston and cylinder, or an electromagneticallyoperated actuator.

Further moving means may be provided to introduce the probe into thecontainer through the opening and may include an actuator of any of thetypes referred to above. Also the probe may be provided with anelectrically insulating cover to close off the container opening duringtesting.

In addition conveyor means may be provided for moving the container intoa testing station and for subsequently moving the container out of thetesting station after leak testing of the container.

The invention also provides a method of leak testing a container, suchas a blow molded container, comprising placing an electricallyconductive probe in communication with the interior of the container byway of an opening of the container, applying a high voltage between theprobe and electrically conductive shield means located externally of thecontainer, and detecting current flow between the probe and the shieldmeans indicating the presence of a hole or weakness in the interveningcontainer wall, characterised in that the high voltage is applied to theprobe, and that relative displacement of the container and the shieldmeans so as to surround at least a major portion of the container withthe shield means is effected prior to application of the high voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood, apparatus inaccordance with the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a first embodiment;

FIG. 2 is a schematic diagram of parts of a second embodiment;

FIGS. 3 and 4 are schematic diagrams of parts of two variantembodiments; and

FIG. 5 is a schematic diagram of parts of a further embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The illustrated apparatus is intended for leak testing plasticscontainers which have been produced by extrusion blow molding, byinjection stretch blow molding or by any similar molding process.Generally the molded plastics containers are supplied along a conveyorline to a testing station in which each container is leak tested by theapparatus before being supplied to a discharge station or a rejectstation, depending on the result of the test.

Referring to FIG. 1, the leak testing apparatus 1 comprises anelectrically conductive probe 2 which is lowered into the interior ofthe container 3 to be tested through an opening 4 of the container bymeans of an actuator 5 which is typically a pneumatic cylinder orelectrical lead screw arrangement. The probe 2 is integrally formed withan electrically insulating cover 6 which seals with the rim 7surrounding the opening 4 of the container when the probe is fullyintroduced into the container. In addition a high voltage source 8applies a high voltage of 30 KV or more to the probe 2 by means of anelectrical connecting lead 9.

At the same time as the probe 2 is introduced into the container 3,shield plates 10, 11 and 12 are moved into positions in which they lieclosely adjacent to the outer walls of the container 3 so that thecontainer is substantially surrounded by these plates 10, 11 and 12.Typically the plates 10 and 11 are shaped to conform to the externalprofile of the container 3 over approximately 180° of arc, whereas theplate 12 is simply a flat plate onto which the container 3 is moved whenit enters the testing station. Respective actuators 13 and 14, which maybe pneumatic cylinders, for example, are provided for moving the plates10 and 11 into the positions in which they surround the container 3, andfor subsequently withdrawing the plates 10 and 11 to enable thecontainer 3 to be moved out of the testing station and a furthercontainer to be introduced into the testing station by means of theconveyor line 30.

In operation the high voltage of 30 KV or more is applied to the probe2, and the current flow along the electrical connecting lead 9 is sensedby a current sensor 15 to determine whether a spark has been generatedbetween the probe 2 and any of the plates 10, 11 and 12 indicating ahole or weakness in the intervening container wall.

Thus the current sensor 15 provides an electrical output indicative ofwhether or not the container wall contains a leak or weakness, and thisoutput may be used to operate a suitable diverter, if required, in orderto divert the container 3 outputted from the testing station eithertowards a discharge zone or towards a reject zone, depending on whetheror not a hole or weakness was detected in the container. The magnitudeof the voltage applied to the probe 2 may be varied depending on whetherit is required simply to detect holes in the container wall or whetherit is also required to detect weaknesses (in which case a higher voltageis required). Since the output signal from the current sensor 15 will beproportional to the spark intensity, it is also possible for a visualoutput to be provided which is indicative of the magnitude of thecurrent sensed and hence indicative of whether a hole or weakness (andthe degree of weakness) has been detected.

FIG. 2 shows, in part, a second embodiment in which each of the plates10, 11 is replaced by an electrically conductive member 16 bearing aseries of electrically conductive foam pads 17, 18 and 19 having aninner profile adapted to the external profile of the container 3. Thesepads 17, 18 and 19 are resilient and thereby enable closer contact withthe outside of the container.

FIG. 3 shows a modification of the probe 2 within the container 3 topermit parts of the probe 2 to be located in close proximity to theinside wall of the container 3 at a plurality of locations duringtesting in order to increase the sensitivity of detection of holes orweaknesses in the container wall. In this case flexible foil strands 20are secured along that part of the probe 2 within the container 3 sothat, when a high voltage is applied to the probe, the foil strands 20will tend to spread out under the influence of static charge as shown inthe figure. Alternatively a probe 2 as shown in FIG. 4 may be used. Sucha probe has a series of electrically conducting rigid vanes 21, only twoof which are shown in FIG. 4, which are pivotally connected to an inneractuating rod 22 within the actuator 2. After introduction of the probe2 into the container 3 with the vanes 21 in an inner pivotal position,the actuating rod 22 may be moved in the direction of the arrow 40 tocause the vanes 21 to be pivoted into any outer pivotal position asshown in the figure.

FIG. 5 shows another version of the apparatus which is adapted not onlyto indicate the occurrence of a hole or weakness in the container wall,but also to provide an indication of the position of the hole orweakness. In this case each shield plate is divided into a series ofelectrically conductive pads 23 each of which is connected to earth(ground) by an associated resistor 24 having a resistance which isdifferent to the resistances of the resistors associated with the otherpads 23. Thus, if a hole or weakness exists in the container wall in thevicinity of a pad 25, for example, a spark 26 will be generated and thiswill produce a voltage V3 at the pad 25 due to the current flowingthrough the resistor R3. Since the resistance values of the variousresistors 24 differ, the current sensed on occurrence of such a sparkwill provide an indication of which of the pads 23 is closest to thehole or weakness detected.

In another possible version of the apparatus, the probe 2 may beprovided with an integral injection nozzle for injecting water mist intothe container 3 at the same time as the probe is introduced into thecontainer. The water mist in the container increases the effectivenessof spark formation.

The electrical discharge testing method possesses a number of advantagesover existing methods, such as pressure decay testing or underwaterbubble testing. Amongst these advantages are:

1. Rapid detection of holes or imperfections since an electricaldischarge occurs in less than a millisecond.

2. Ability to locate very small holes of less than 0.1 mm, and even todetect thin areas which might potentially become holes.

3. Ability to provide an indication of approximate hole position ifrequired.

4. Increased reliability since delicate transducers are not required.

5. Sterility which is particularly advantageous in the case ofcontainers for medical application.

I claim:
 1. Apparatus for leak testing a container comprising anelectrically conductive probe (2) for communicating with an interior ofthe container (3) by way of an opening (4) of the container,electrically conductive shield means (10, 11, 12; 16, 17, 18, 19; 23)closely surrounding the container and movable between open and closedpositions, electrical connection means (9) for application of a highvoltage relative to the shield means, and sensor means (15) fordetecting current flow between the probe and the shield means indicatingthe presence of at least one of a hole and a weakness in an interveningwall of the container, and moving means (13, 14) for moving the shieldmeans between the open position permitting introduction or removal ofthe container into or from the shield means and the closed position inwhich at least a major portion of the container is closely surrounded bythe shield means.
 2. Apparatus according to claim 1, wherein the probe(2) has a form which ensures that parts of the probe are located inclose proximity to an inside wall of the container (3) at a plurality oflocations during testing.
 3. Apparatus according to claim 2, wherein theprobe (2) includes a plurality of electrically conductive strands (20)capable of spreading out under an influence of a static charge. 4.Apparatus according to claim 1, wherein the shield means comprises oneof electrically conductive plates (10, 11, 12), foam pads (17, 18, 19)and brushes.
 5. Apparatus according to claim 1, wherein the shield meanscomprises a plurality of parts (23) which are electrically insulatedfrom one another and connected to earth by respective resistance means(24) so as to enable an approximate location of at least one of the holeand weakness to be detected by the sensor means (15).
 6. Apparatusaccording to claim 1, wherein the moving means comprises a plurality ofmechanical actuators (13, 14) for moving associated parts of the shieldmeans (10, 11, 12; 16, 17, 18, 19; 23) into close proximity with anoutside wall of the container (3).
 7. Apparatus according to claim 1,wherein further moving means (5) is provided to introduce the probe (2)into the container (3) through the opening (4).
 8. Apparatus accordingto claim 1, wherein the probe (2) is provided with an electricallyinsulating cover (6) to close off the container opening (4) duringtesting.
 9. Apparatus according to claim 1, wherein conveyor means (30)is provided for moving the container into a testing station and forsubsequently moving the container (3) out of the testing station afterleak testing of the container.
 10. A method of leak testing a containercomprising a step of placing an electrically conductive probe (2) incommunication with an interior of the container (3) by way of an opening(4) of the container, a step of closely surrounding at least a majorportion of the container with electrically conductive shield means (10,11, 12; 16, 17, 18, 19; 23) in a closed position, a step of detectingcurrent flow between the probe and the shield means indicating thepresence of at least one of a hole and a weakness in an intervening wallof the container, and a step of moving the shield means from the closedposition into an open position permitting introduction or removal of thecontainer into or from the shield means.
 11. Method according to claim10, wherein said container is a blow molded container.